How can Bevy's documentation be improved?

This issue is a more specific form of #8770, which has been open for over nine months with no apparent resolution.

The caching example for SystemState in the documentation shows the use of a function-local struct to cache a SystemState object. However, caching a SystemState has gotten easier since this documentation was written - with the addition of ExclusiveSystemParam and its implementation on &'a mut SystemState<P>, we can have the ECS handle caching for us.

The documentation should be updated to reflect this.